1. Field of the Invention
The present invention relates to controllable timing circuitry for digital electronic musical instruments.
2. Description of the Prior Art
In electronic musical instruments, many functions other than the determination of the note fundamental frequency are carried out at clock rates that preferably are independently adjustable. For example, the amplitude envelope of the generated tone may be established by a set of amplitude scale factors that are supplied at a selectable rate which establishes the attack, decay, sustain and release duration of each note. Other functions include vibrato and tremolo modulation effects, arpeggio and glissando, and various sliding formant effects.
An oscillator of adjustable frequency may be used as the time standard for rate control of such functions. However, the use of an analog oscillator in a digital tone generating system requires some artifice to convert the analog clock signals to a form usable with digital circuitry. One such artifice involves the use of an analog square wave oscillator and a counter that is incremented or decremented each time the oscillator square wave output changes sign. The counter contents may be used e.g., to address a memory storing tonal modification scale factors. The clock rate can be controlled in analog fashion by changing some oscillator component value, such as the capacitance or resistance in an RC time constant circuit.
Among the disadvantages of such a hybrid analog-digital timing system is the problem of controlling the clock rate in response to some digital system parameter. For example, it may be desirable to vary the attack/release amplitude envelope duration as a function of the selected note or octave, so that the attack or release time for notes of lower octaves is longer than for notes of higher octaves. If the fundamental frequency of the generated note is specified by a digital number, digital-to-analog converter circuitry may be required to enable adjustment of an analog oscillator clock rate in response to the selected frequency number.
Thus is it an object of the present invention to provide a digital timing system for use in a digital electronic musical instrument. The inventive system advantageously is used as the time base for modulation and special effects other than note frequency determination.
A further object is to provide such a digital timing system in which digital rate control readily can be implemented.